<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    </head>
    <body>
        <h1>我的第一段 JavaScript</h1>
        <p>请输入数字。如果输入值不是数字，浏览器会弹出提示框。</p>
        <input id="demo" type="text">
        <script>
            function myFunction() {
                var x = document.getElementById("demo").value;
                if (x === "" || isNaN(x)) {
                    alert("Not Numeric");
                }
            }
        </script>
        <button type="button" onclick="myFunction()">点击这里</button>
    </body>
</html>
